This invention relates to a thermal radiation detector for detecting radiation in a given wavelength range, comprising detecting means having a temperature-dependent characteristic for detecting thermal energy, and, with respect to the direction of incidence of thermal radiation to be detected, a dielectric layer and an electrically resistive layer preceding the detecting means, wherein the electrically resistive layer is disposed immediately adjacent the dielectric layer and has a substantial net effective resistance per square such that the resistive layer absorbs a substantial proportion of incident radiation in said wavelength range and wherein the optical thickness of the dielectric layer is substantially one quarter of a selected wavelength in the wavelength range.
A problem with thermal radiation detectors is to absorb sufficient of the incident radiation. For example, pyroelectric materials suitable for good-quality detectors typically have rather low absorption over at least part of the wavelength range for which it is desirable to provide such detectors; furthermore, the thinner the detector (a feature which is desirable to reduce particularly thermal capacitance as well as its thermal conductance to the surroundings), the lower will be the overall absorption of the incident radiation by the pyroelectric material. A conventional way of improving absorption has been to apply a "black" (blackening); however, appropriate materials have the disadvantages of being awkward to apply and having poor adhesion.
A detector as set forth in the opening paragraph wherein the detecting means comprises an element of pyroelectric material is known from British Patent Application No. 8508204 which corresponds to U.S. Pat. No. 4,704,534 whose disclosure is incorporated herein by reference.
In one form of infrared detector described therein, the dielectric film is constituted by a flexible film which supports the detector means and which is substantially transparent in the wavelength range 8-14 micrometers and is approximately one quarter of a selected wavelength at which it is desired that the absorption of incident radiation by the resistive layer should be a maximum. With the detecting means thermally well coupled to the resistive layer, and by virtue of the flexible film, which provides the mechanical advantage to the detector of low susceptibility to microphony and low lateral thermal conductance, absorption of incident radiation is considerably improved. In one embodiment of the detector, the resistive layer is disposed between the flexible film and the detecting means and its resistance per square is adapted in dependence on the refractive index of the flexible film to optimise the absorption of incident radiation by the layer in the region of the selected wavelength.
In another embodiment, the resistive layer is disposed on the side of the flexible film remote from the detecting means and a reflecting electrically conductive layer interposed between the flexible film and the detecting means.
These detectors provide a good overall incident radiation absorption characteristic in the wavelength range of interest with a very high absorption coefficient at the selected wavelength. The absorption curve is strongly peaked however and either side of this wavelength the value of absorption falls sharply. The rate at which it falls increases with the refractive index of the dielectric layer.
Whilst this absorption response is acceptable for certain applications of the detector, for other applications it would be desirable for the detector to exhibit an optimisation of the absorption characteristic for a wider waveband and it is an object of the invention to provide such a detector,